1. Field of the Invention
The present invention relates to fluorescent lamps of the preheat or heated filament type and to electronic ballasts of the type having a filament power supply including a multiple output DC to DC converter.
2. Background Art
The use of fluorescent lamps has become widespread. The typical fluorescent lamp is composed of a glass tube containing an inert gas and a small amount of mercury. Phosphors coat the inside of the glass tube, and each end of the glass tube includes an electrode. In operation, a ballast provides current to the electrodes. A traditional ballast is a special transformer that uses electromagnetic principles to generate operating and starting voltages for fluorescent lamps. An electronic ballast uses electronics to achieve the same result. In operation, electrons migrate across the length of the tube, and excite the mercury atoms which are in a gaseous state. The arc releases photons in the ultraviolet band. The photons excite the phosphors that coat the inside of the glass tube, and the phosphors emit visible light. Fluorescent lamps are very efficient during operation. Before a fluorescent lamp can operate as described above, the lamp must be started, that is, the length of tube must be made conductive. There are several existing techniques for starting a fluorescent lamp.
One technique for starting a fluorescent lamp involves the use of electrodes that include filaments. Each electrode is composed of two conductive pins that connect to a filament wire including tungsten and boron. Preheating the filament at each end of the fluorescent lamp tube boils electrons from the filament to ionize the gas inside the tube. The ionized gas inside the glass tube is conductive, and needs a voltage across the electrodes to establish an electrical arc. Using preheating techniques for the filaments increases lamp life, enhances dimming performance and enhances cold operation performance.
Another technique for starting a fluorescent lamp is known as instant start. In instant start fluorescent lamps, a very high initial voltage is applied across the electrodes which are typically single pin electrodes. The high voltage causes a corona discharge where the gas inside the glass tube is quickly ionized and an electrical arc is established. Although instant start is used in many fluorescent lamp applications, some fluorescent lamp applications demand that preheating techniques are utilized. Further, some applications continually heat the filaments even after establishing the electric arc.
Electronic ballasts have been used in fluorescent lamps of the preheat and heated filament type. The electronic ballasts typically include a filament power supply to provide filament heating power and to provide operating high voltage. Various approaches have been taken for providing the filament heating power.
One existing filament power supply for an electronic ballast uses a steel core transformer as a low frequency transformer to provide filament heating power. The transformer is physically large due to operation at 50 Hz, 60 Hz, or 400 Hz. Primary magnetizing losses and losses in the large turn windings make this approach electrically inefficient. In the event that a lamp filament is shorted, the short is reflected to the transformer primary side, thus shorting the ballast input. Recyclable thermal protection, thermal fuses or fuses are usually employed to prevent overheating of the ballast during this condition.
Another existing filament power supply for an electronic ballast uses a DC output flyback converter. The flyback converter topology reduces component count, and accommodates multiple outputs. The use of high frequency power conversion reduces the size and weight of the power transformer. The electrical efficiency is improved over the filament power supply using a steel core transformer.
Use of a high frequency switch mode converter to generate filament voltages has historically not been practical due to the circuit complexity and cost of such an approach. Recent advances in technology make this approach more viable. Accordingly, electronic ballasts of the type having a filament power supply including a DC output flyback converter are desirable for some preheat or heated filament type fluorescent lamp applications.
A particular problem faced in the fluorescent lamp industry is violent lamp end of life failure in certain applications caused by overheating of a broken or disconnected filament. Another particular problem faced in the fluorescent lamp industry is lamp to contact high voltage arcing caused by a loose or misinstalled lamp or an excessively worn lamp socket, and an excess voltage. Another particular problem faced in the fluorescent lamp industry is that heavily carbonized lamp holders may smolder during operation of the lamp.
To address these problems, some existing approaches detect when an arcing event is taking place and then shutdown the ballast high voltage constant current generator that generates the operating voltage. Such an approach, by design, requires that an arc occur so that it can be detected. Also, these approaches may fail to detect a smoldering lamp holder resulting in the lamp continuing to operate despite the potentially problematic situation. Background information relating to fluorescent lamps may be found in U.S. Pat. Nos. 4,668,946; 4,870,529; 4,949,013; 5,574,335; 5,703,441; 5,729,096; 5,869,935; 5,952,832; 6,140,771; and 6,175,189. Background information relating to current transformers may be found in Billings, Keith, Switchmode Power Supply Handbook, McGraw-Hill, 1999.
For the foregoing reasons, there is a need for an improved electronic ballast having a filament power supply including a DC converter for use with fluorescent lamps of the preheat or heated filament type that utilizes improved filament detection techniques suitable for detecting disconnected filaments, detecting other problems that appear as an open circuit such as loose or misinstalled lamps, or detecting the smoldering of a heavily carbonized lamp holder.
It is, therefore, an object of the present invention to provide a fluorescent lamp electronic ballast for use with fluorescent lamps of the preheat or heated filament type that utilizes a current transformer in a multiple output DC to DC converter.
In carrying out the above object, a fluorescent lamp electronic ballast for use with fluorescent lamps of the preheat or heated filament type is provided. The electronic ballast comprises a filament power supply, a sensing circuit, and a controller. The filament power supply includes a high voltage constant current generator for generating lamp operating voltages, and a multiple output DC to DC converter for providing filament heating power. The converter includes a primary winding and a plurality of secondary windings for connection to a plurality of lamp filaments. Each secondary winding is connected to a diode/capacitor combination to produce a DC voltage across the capacitor by action of the converter. This DC voltage is applied across each respective filament.
High frequency pulses of current are present in a loop composed of the secondary winding, diode and capacitor. These pulses of current are unidirectional due to the diode and action of the converter. A current transformer primary is inserted in any position in the high frequency loop. Switching the converter at high frequency allows the size of the current transformer to be minimized.
A secondary of the current transformer is connected to a diode that feeds a parallel combination of a resistor and a capacitor. The diode feeds the unidirectional secondary current into the resistor to produce a voltage analogous to current. The capacitor then peak charges this voltage to produce a DC voltage proportional to the RMS current of high frequency loop of the converter.
It is appreciated that the current measured with this technique is not exactly equivalent to the DC current flowing in the filament as some of the high frequency current is circulated through the capacitor that is in parallel with the filament. For this application the current measurement needs only to be proportional to the actual filament current.
It is appreciated that the sensing circuit may take any suitable form. For example, the high frequency loop current transformers may each have their own current transformer secondary winding connected to a peak detector, or the high frequency loop current transformers may share a single shared current transformer secondary winding connected to a peak detector. Individual current transformer secondary windings allow the sensing circuit to gather individual values that represent the high frequency loop current in each individual secondary winding of the converter. A single shared current transformer secondary winding allows the sensing circuit to gather a single value that represents the sum or total high frequency loop current of the secondary windings of the converter. Further, a single shared current transformer secondary winding may be used together with current transformer primary windings having varying numbers of turns with respect to each other such that is still possible for the sensing circuit or controller to determine individual values that represent the high frequency loop current in each individual secondary winding of the converter.
Further, it is to be appreciated that the controller may take any suitable form such as a microprocessor or microcontroller, or even discrete components arranged to provide the needed control. And further, the way that the controller is connected to the sensing circuit may take any suitable form, such as any number of individual inputs, multiplexed inputs, etcetera. It is appreciated that the structure of the filament power supply and location of the current transformers provides sensed signals that are indicative of the presence of open circuits in the filament circuits, that is, indicative of disconnected filaments, loose or misinstalled lamps, etcetera. Because the current transformers monitor filament current, the sensed signals may also be examined to detect the presence of short circuits or heavily carbonized lamp holders.
Preferably, the controller shuts down the high voltage generator when the high voltage generator is operating and the signals from the sensing circuit indicate an open circuit fault. Further, preferably, the controller prevents the operation of the high voltage generator when the high voltage generator is not operating and the signals from the sensing circuit indicate an open circuit fault.
In some embodiments, the controller controls the high voltage generator based on absolute measurements for filaments. That is, it is possible to compare values that represent the high frequency loop currents (including values that represent individual currents and values that represent sums of individual currents) to fixed reference values to determine filament status. These absolute comparisons are useful in many applications where the electronic ballast is designed for a specific lamp. The absolute measurements may be compared to either a fixed reference or a variable reference. On the other hand, it is possible to compare values (when there is more than one value) that represent the high frequency loop currents to each other to determine filament status. These relative comparisons are useful for universal ballasts that are not designed for any one specific lamp, and are also useful for detecting a smoldering condition due to a heavily carbonized lamp holder.
Further, in carrying out the present invention, a fluorescent lamp electronic ballast for use with single and dual fluorescent lamp configurations including fluorescent lamps of the preheat or heated filament type is provided. The electronic ballast comprises a filament power supply including a high voltage constant current generator for generating lamp operating voltages, and a multiple output DC to DC converter for providing filament heating power. The converter includes a primary winding and a plurality of secondary windings for connection to a plurality of lamp filaments include a first filament, second and third filaments connected in parallel, and a fourth filament. Each secondary winding is connected to a diode/capacitor combination to produce a DC voltage across the capacitor by action of the converter. This DC voltage is applied across each respective filament.
High frequency pulses of current are present in a loop composed of the secondary winding, diode and capacitor. These pulses of current are unidirectional due to the diode and action of the converter. A current transformer primary is inserted in any position in the high frequency loop. Switching the converter at high frequency allows the size of the current transformer to be minimized.
A secondary of the current transformer is connected to a diode that feeds a parallel combination of a resistor and a capacitor. The diode feeds the unidirectional secondary current into the resistor to produce a voltage analogous to current. The capacitor then peak charges this voltage to produce a DC voltage proportional to the RMS current of high frequency loop of the converter.
It is appreciated that the current measured with this technique is not exactly equivalent to the DC current flowing in the filament as some of the high frequency current is circulated through the capacitor that is in parallel with the filament. For this application the current measurement needs only to be proportional to the actual filament current.
Preferably, the controller shuts down the high voltage generator when the high voltage generator is operating and the signals from the sensing circuit indicate an open circuit fault. Further, preferably, the controller prevents the operation of the high voltage generator when the high voltage generator is not operating and the signals from the sensing circuit indicate an open circuit fault.
Preferably, the controller discriminates between single and dual fluorescent lamp configurations based on a signal from the sensing circuit corresponding to the second and third filaments.
Still further, in carrying out the present invention, a fluorescent lamp electronic ballast for use with single and dual fluorescent lamp configurations including fluorescent lamps of the preheat or heated filament type is provided. The electronic ballast includes a filament power supply including a high voltage constant current generator for generating lamp operating voltages, and a multiple output DC to DC converter for providing filament heating power. The converter includes a primary winding and a plurality of secondary windings for connection to a plurality of lamp filaments include a first filament, second and third filaments connected in parallel, and a fourth filament. Each secondary winding is connected to a diode/capacitor combination to produce a DC voltage across the capacitor by action of the converter. This DC voltage is applied across each respective filament.
High frequency pulses of current are present in a loop composed of the secondary winding, diode and capacitor. These pulses of current are unidirectional due to the diode and action of the converter. A current transformer primary is inserted in any position in the high frequency loop. Switching the converter at high frequency allows the size of the current transformer to be minimized.
A secondary of the current transformer is connected to a diode that feeds a parallel combination of a resistor and a capacitor. The diode feeds the unidirectional secondary current into the resistor to produce a voltage analogous to current. The capacitor then peak charges this voltage to produce a DC voltage proportional to the RMS current of high frequency loop of the converter.
It is appreciated that the current measured with this technique is not exactly equivalent to the DC current flowing in the filament as some of the high frequency current is circulated through the capacitor that is in parallel with the filament. For this application the current measurement needs only to be proportional to the actual filament current.
The controller is programmed to preheat the filaments, and to measure a sensing circuit signal corresponding to the second and third filaments. The controller discriminates between single and dual fluorescent lamp configurations based on the signal from the sensing circuit corresponding to the second and third filaments.
In a preferred embodiment, the controller is further programmed, in the dual lamp configuration, to measure a sensing circuit signal corresponding to the first filament, and measure a sensing circuit signal corresponding to the fourth filament. The controller determines a presence of an open circuit fault based on a comparison of a sum of the sensing circuit signals corresponding to the first and fourth filaments and the sensing circuit signal corresponding to the second and third filaments. The controller prevents operation of the high voltage generator in the presence of an open circuit fault.
In a preferred embodiment, the controller is further programmed, in the single lamp configuration, to measure a sensing circuit signal corresponding to the first filament, and measure a sensing circuit signal corresponding to the fourth filament. The controller determines a presence of an open circuit fault based on the sending circuit signals corresponding to the first and fourth filaments. The controller prevents operation of the high voltage generator in the presence of an open circuit fault.
In a preferred embodiment, the controller is further programmed to determine a presence of an open circuit fault based on a sum of the sensing circuit signals corresponding to the first and fourth filaments. The controller prevents operation of the high voltage generator in the presence of an open circuit fault.